Nanocomposite material and method of manufacturing the same

ABSTRACT

A nanocomposite material and a method of manufacturing the same are disclosed. The nanocomposite material includes a plurality of nanoparticles coated with a metal oxide, and a matrix of the metal oxide immobilizing the nanoparticles that are dispersed therein. The nanocomposite material is manufactured such that macro- or micro-scale cracks are prevented or effectively prevented, light stability is enhanced over a light-emitting period, and light brightness is improved.

This application is a divisional application of U.S. application Ser.No. 11/566,253, filed on Dec. 4, 2006, which claims priority to KoreanPatent Application No. 10-2006-0016675, filed on Feb. 21, 2006, and allthe benefits accruing therefrom under 35 U.S.C. §119, the contents ofwhich in their entirety are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nanocomposite material and a methodof manufacturing the same. More particularly the present inventionrelates to a nanocomposite material whose internal defects are reducedor eliminated and light stability is enhanced, and a method ofmanufacturing the same.

2. Description of the Related Art

An example of a nanocomposite is a composite material in whichnanoparticles such as quantum dots are densely arranged within alight-transparent matrix material consisting of a metal oxide or apolymer material.

Ligand-exchange processes have been used to manufacture such ananocomposite material. In particular, a surface ligand of an organicmolecule that is chemically coordinated on the surface of the quantumdots obtained by a wet chemical process is exchanged with a hydrophilicligand having high affinity for a polar solvent such as water or analcohol. For example, an organic compound such as 4-methylaminopyridine(DMAP) or another polymeric compound is added and the surface ligand ofan organic molecule is exchanged with a hydrophilic ligand. Then thequantum dots, whose surfaces are modified, are mixed in a precursorsolution, and the mixture is subjected to a sol-gel reaction and then adrying process, thereby obtaining a nanocomposite material impregnatedwith the quantum dots as a hardened solid.

FIGS. 1A and 1B are photographs of an existing nanocomposite materialduring the drying process, in which cracks grow when the nanocompositematerial is being dried due to a non-symmetrical drying rate between thesurface and the inner body of the nanocomposite, which eventually cracksinto pieces. The binding strength and affinity between a matrix materialand quantum dots impregnated therein is so insufficient that cracksoccur frequently in the nanocomposite material as the solvent in the solphase is evaporated during the drying process. Such cracks formed in thenanocomposite material, for example, reduce the efficiency forextracting light, provide a penetrating path for harmful materials suchas oxygen/moisture thereby reducing the emitting property of thenanoparticles, and decrease the durability of the nanocompositematerial.

To address these problems, the drying process can be somewhat delayed byadding a drying releasing agent such as dimethylformamide (DMF) to aprecursor solution of the matrix. However, for such a method, impuritiesare inevitably incorporated into the nanocomposite material, anddisruptive behavior can be induced in the nanocomposite material as theextra releasing agent is evaporated.

BRIEF SUMMARY OF THE INVENTION

The present invention includes providing a nanocomposite material whoseinternal defects such as cracks are reduced or eliminated and whoselight-emitting stability is enhanced, as well as a method ofmanufacturing the same.

According to an exemplary embodiment of the present invention, ananocomposite material includes a plurality of nanoparticles coated witha metal oxide, and a matrix of the metal oxide immobilizing thenanoparticles that are dispersed therein.

The matrix may be formed from a network structure of the metal oxide.

The metal oxide may be SiO₂ or TiO₂.

The nanoparticles may include Group II-Vi semiconductor compoundsselected from the group consisting of CdSe, CdTe, CdS, ZnSe, ZnTe, ZnS,InP, GaP, and GaInP₂.

The nanoparticles may include metal particles selected from the groupconsisting of Au, Ag, Fe, and Co, or an oxide particle of the selectedmetal.

According to another exemplary embodiment of the present invention, amethod of manufacturing a nanocomposite material includes coating thesurface of synthesized nanoparticles with a metal oxide to modify thesurface; conducting a sol-gel reaction in a precursor solution in whichthe surface-modified nanoparticles are mixed with a precursor materialof the metal oxide; and drying and gelling the precursor solution toform a solid matrix.

The method may also include synthesizing the nanoparticles.

The nanoparticles can be synthesized through a wet-chemical process,that includes injecting at least one precursor material into an organicsolvent capable of being coordinated, and growing crystals intonanoparticles having a uniform size.

Modifying the surface includes mixing the nanoparticles, an alkalicatalyst for synthesizing a metal oxide, and a precursor material of themetal oxide in an organic solvent in which a surfactant is dispersed,and conducting a condensation reaction to synthesize the metal oxide onthe surface of the nanoparticles. The surfactant may be a non-ionicsurfactant that has an ether group as a hydrophilic group and an alkylgroup as a hydrophobic group.

The sol-gel reaction is conducted in a precursor solution in whichcoated nanoparticles, a precursor material, and an alkali catalystmediating a covalent bond between the nanoparticles and the precursormaterial are mixed in predetermined ratios in an aqueous polar solvent.

Drying includes exposing the precursor solution to an inert gasatmosphere or heating the precursor solution at a constant temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill be more clearly understood from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIGS. 1A and 1B are photographs showing a disruptive phenomenon of anexisting nanocomposite material due to a crack caused during drying;

FIGS. 2A through 2C are schematic illustrations of an exemplaryembodiment of a method of manufacturing a nanocomposite materialaccording to the present invention;

FIGS. 3A through 3D are schematic illustrations of an exmemplaryembodiment of a process of modifying a surface of a nanoparticleaccording to the present invention;

FIGS. 4A through 4C are schematic illustrations of an exemplaryembodiment of a sol-gel reaction process and a drying process accordingto the present invention;

FIGS. 5A and 5B are transmission electron microscope (TEM) images atdifferent magnifications of nanoparticles on which a silica coating wasformed;

FIG. 6A is a photograph of an exemplary embodiment of a nanocompositematerial according to the present invention;

FIG. 6B is a photograph illustrating a light-emitting state of thenanocomposite material of FIG. 6A when excited light was irradiated onthe nanocomposite material; and

FIG. 7 is a graph comparing the light-emitting properties of anexemplary embodiment of a nanocomposite material according to thepresent invention with that of an existing nanocomposite material.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which exemplary embodimentsof the present invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the exemplary embodiments set forth herein. Rather, these exemplaryembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like reference numerals refer to like elementsthroughout.

It will be understood that when an element is referred to as being “on”another element, it can be directly on the other element or interveningelements may be present therebetween. In contrast, when an element isreferred to as being “directly on” another element, there are nointervening elements present. As used herein, the term “and/or” includesany and all combinations of one or more of the associated listed items.

It will be understood that, although the terms first, second, third, andthe like may be used herein to describe various elements, components,regions, layers and/or sections, these elements, components, regions,layers and/or sections should not be limited by these terms. These termsare only used to distinguish one element, component, region, layer orsection from another element, component, region, layer or section. Thus,a first element, component, region, layer or section discussed belowcould be termed a second element, component, region, layer or sectionwithout departing from the teachings of the present invention.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” or “includes” and/or “including” when used in thisspecification, specify the presence of stated features, regions,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,regions, integers, steps, operations, elements, components, and/orgroups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

Throughout the specification, the nanoparticles refer to nanoparticleswhich have specific chemical, optical, electrical, and/or magneticproperties, depending on their size. The nanoparticles can includequantum scale nanoparticles, i.e., those having an average size of lessthan or equal to about 10 nanometers (nm), of at least one metalselected from the group consisting of Au, Ag, Fe, and Co, or an oxideparticle of one or more of the foregoing metals (e.g., Fe₂O₃). Thenanoparticles can include quantum dots comprising one or more GroupII-VI semiconductor compounds. In an exemplary embodiment, the quantumdots comprise one or more semiconductor compound selected from the groupconsisting of CdSe, CdTe, CdS, ZnSe, ZnTe, ZnS, InP, GaP, and GaInP₂.The nanoparticles can have a homogeneous single structure or acore-shell double structure. Each core and shell in the core-shelldouble structure may comprise a different semiconductor compound, metal,and/or metal oxide. In an exemplary embodiment, the core and shell inthe core-shell structure comprises one or more selected from the groupconsisting of CdSe, CdTe, CdS, ZnSe, ZnTe, ZnS, InP, GaP, Au, Ag, Fe,Co, Fe₂O₃, FePt, and GaInP₂. The energy band gap of the shell materialmay be greater than that of the core material. The quantum dots emitenergy according to an inherent energy band gap when receiving lightfrom a light source, and reach an excited state. Therefore, the energyband gap can be controlled by controlling the size of the quantum dots,and thus light having various wavelengths spanning the visible lightspectrum as well as ultraviolet light can be observed.

FIGS. 2A through 2C are schematic illustrations of an exemplaryembodiment of a method of manufacturing a nanocomposite materialaccording to the present invention. Referring to FIGS. 2A through 2C,the method of manufacturing a nanocomposite material comprisessynthesizing nanoparticles 10 (FIG. 2A), modifying the surface of thenanoparticles 10 by forming a matrix coating layer 30 on the surface ofthe nanoparticles 10 (FIG. 2B), conducting a sol-gel reaction in aprecursor solution in which the surface-modified nanoparticles 10 aremixed with a precursor material of the matrix, and drying the precursorsolution to form a solid matrix 50 in which the nanoparticles 10 areimpregnated in the solid state (FIG. 2C).

The quantum dots, as an example of the nanoparticles 10, can besynthesized using a wet chemical process. In an exemplary wet chemicalprocess, a precursor material is incorporated into an organic solvent,which acts as a dispersant, and the growth of the nanoparticles iscontrolled by coordinating the organic solvent on the surface ofnanoparticle crystals. For example, in order to obtain quantum dots ofCdSe, a precursor material such as dimethyl cadmium ((CH₃)₂Cd) ortrioctylphosphine selenide (TOPSe) is incorporated into an organicsolvent that uses trioctylphosphine oxide (TOPO) as a surfactant,thereby forming crystals, and the crystals are maintained for a specifictime at a high temperature so that the crystals can grow to have auniform size. In one embodiment, nanoparticles of CdSe/ZnS, which aresurface capped with an organic molecule such as TOPO are produced.

As can be seen in FIG. 2A, the surface of the nanoparticles 10 obtainedusing such a wet chemical process is coordinated with an organic ligand15. The process of modifying the surfaces of the nanoparticles 10 asdescribed below is conducted so that the nanoparticles 10 can be firmlyincorporated into a network of a metal oxide matrix 50. As can be seenin FIG. 2B, in modifying the surface of the nanoparticles 10, thesurfaces of the nanoparticles 10 are coated with the metal oxide thatconstitutes the matrix 50 (coating layer: 30). By way of example, in anexemplary embodiment, when the matrix 50 is formed from a network ofsilica (SiO₂), the surface of the nanoparticles 10 is coated withsilica. FIGS. 3A through 3D are schematic illustrations of an exemplaryembodiment of a process of modifying a surface of the nanoparticles 10according to the present invention. First, as can be seen in FIG. 3A, asurfactant is dispersed in an organic solvent. In an exemplaryembodiment, the surfactant is a non-ionic surfactant that has an ethergroup as a hydrophilic group, and has an alkyl group as a hydrophobicgroup. For example, about 50 mole percent (mol %) ofpolyoxyethylene(5)nonylphenyl ether (trade name: IGEPAL CO-520, which ishereinafter referred to IGEPAL) is dispersed in cyclohexane. A uniformdispersion may be achieved in the solvent using sonication. FIG. 3Aschematically depicts the state of IGEPAL dispersed in the cyclohexanesolvent. As seen in FIG. 3B, nanoparticles are subsequently mixed in theIGEPAL solution. For example, the solution in which CdSe quantum dots (1milligram of quantum dots per milliliter of cyclohexane solvent) aredissolved is mixed in the IGEPAL solution. Next, as seen in FIG. 3C, analkaline catalyst is added to the mixed solution of the IGEPAL andnanoparticles. For example, a 29.4 volume percent solution of aqueousammonia (NH₄OH) is added to the mixed solution to form a transparentemulsion. The added aqueous ammonia solution is present in the form ofdroplets in the mixed solution. As seen in FIG. 3C, the droplets of theaqueous ammonia solution are surrounded by the IGEPAL, and thus areuniformly dispersed in the mixed solution. The droplets provide areaction environment for synthesizing silica as nanoparticles 10 areincorporated into the droplets.

Next, as seen in FIG. 3D, tetraethylorthosilicate (TEOS), which is asilica precursor material, is added to the emulsion, and the mixture isallowed to react for about 48 hours. The added precursor molecules arebound together while surrounding the nanoparticles 10, thereby growingto form a silica-coating layer. More particularly, the aqueous ammoniasolution present in droplets acts as a catalyst to direct hydroxy groups(—OH) to the precursor molecules. The precursor molecules are boundtogether through a condensation reaction in which the hydroxy ions aredehydrated, thereby forming a silica coating layer in a gel state. Thediameter of the nanoparticles 10 capped by the silica is about 20 toabout 50 nm at the time that the reaction is complete.

After coating the surface of the nanoparticles with silica as describedabove, a matrix of a gelled metal oxide is formed so that thenanoparticles 10 can be immobilized therein. First, as shown in FIG. 4Aand FIG. 4B, the nanoparticles 10, which are encapsulated by the coatinglayer 30 are extracted from the solution through centrifugation or thelike, and are dispersed in an alcoholic polar solvent 50′ such asethanol at a concentration of about 30 milligrams of encapsulatednanoparticles per ml of alcohol. Next, a silica precursor material,which can be tetramethoxysilane, is added to the ethanol solvent inwhich the nanoparticles 10 are dispersed. An aqueous ammonia solution(NH₄OH) is added thereto as an alkaline catalyst. The amount of theadded ammonia solution can be increased or decreased within a properrange (e.g., about 5 microliters to about 20 microliters) according tothe desired reaction rate. The aqueous ammonia solution acts as acatalyst to direct hydroxy groups (—OH) to the precursor molecules. Theprecursor molecules are bound together through a condensation reactionin which the hydroxy ions are dehydrated, thereby forming a networkstructure of silica. The nanoparticles 10 are immobilized in the silicamatrix in a gel state through a sol-gel reaction accompanied by thecondensation reaction. In other words, when the solution in which thenanoparticles 10 are dispersed is exposed to an inert gas atmosphere, aphase change occurs from a liquid state to a gel state while the ethanolsolvent evaporates over time.

When the silica matrix in the gel state is exposed again to an ambientenvironment for a sufficient time, any remaining ethanol evaporates andis cured, thereby leaving a nanocomposite material 100 as seen in FIG.4C. In order to promote the curing reaction during the drying process,the solution in which the nanoparticles 10 are dissolved may be heatedat a specific temperature (e.g., less than or equal to about 100 degreesCelsius). However, the heating temperature should be limited so as toprevent internal cracks due to quick drying. Such a drying process maybe also performed in a restricted humidity environment since, in anatmosphere having excessively high humidity, the evaporation of thealcoholic solvent is delayed, and thus it is not easy to obtain a matrixin a sufficiently dried state. In an advantageous feature of the processdisclosed herein, macro- or micro-scale cracks in the drying process,which are caused due to lack of affinity between the nanoparticles andthe matrix, are suppressed by introducing a surface modifying processthat coats the nanoparticles with a metal oxide identical with thematrix material prior to the drying process.

FIG. 5A is a transmission electron microscope (TEM) image ofnanoparticles having a core/shell (CdSe/ZnS) structure coated withsilica (SiO₂). FIG. 5B is a high-resolution TEM photograph of thenanoparticles of FIG. 5A. In FIG. 5B, portions that have a crystallattice structure and are dark denote nanoparticles (labeled as “QD”),and portions that surround the nanoparticles (QD) and are lighter denotethe silica coating layer (labeled as “SiO₂”) surrounding thenanoparticles (QD). FIG. 6A is a photograph of an exemplary embodimentof a nanocomposite material according to the present invention, and FIG.6B is a photograph illustrating a light-emitting state of thenanocomposite material of FIG. 6A when ultraviolet (UV) light of aspecific wavelength was irradiated on the nanocomposite material.

FIG. 7 is a graphical comparison of the light-emitting stability of anexemplary embodiment of a nanocomposite material according to thepresent invention and that of an existing nanocomposite material. FIG. 7shows the normalized intensity of light over time when irradiating UVlight at a wavelength of 398 nm. The normalized intensity of lightdenotes a value normalized based on the light intensity at initiallight-emission. As can be seen in FIG. 7, an existing nanocompositematerial (“ND in Silica cracked”) shows that the normalized lightintensity is unstable since the light intensity is different, with awide range, over the light-emitting time. Such an emitting propertyprobably results from the phenomenon that the state of light extractionis distorted by macro- or micro-sized cracks generated in thenanocomposite material.

Meanwhile, a nanocomposite material that adopted polydimethylsiloxane(PDMS) as a matrix (ND in PDMS) exhibits more stable light-emittingproperties than the existing nanocomposite material in which cracks wereformed, and shows a tendency that its normalized light intensity iscontinuously decreased over light-emitting time. For nanocompositematerials comprising a polymer matrix such as PDMS, urethane, and epoxy,the matrix that is originally amorphous and light-transparent isgradually crystallized thereby becoming opaque as the irradiation timefrom an excited light source increases. This results in a reduction ofthe normalized light intensity over time. For a nanocomposite materialhaving a silica glass-based matrix made according to an exemplaryembodiment of the present invention (ND in Silica crackless), althoughthere is a tendency that the normalized light intensity decreases overlight-emitting time, the extent of this tendency is lower than that ofthe polymer matrix sample (ND in PDMS). This nanocomposite material (NDin Silica crackless) also has more stable light-emitting properties, andexhibits a generally enhanced normalized light intensity.

The nanocomposite material of the present invention can be widelyapplied in various fields such as light devices, display devices,sensing devices, and the like. For example, it can be used as alight-emitting diode that generates visible light in a specificwavelength range, for example, as carriers are injected into the quantumdots through electrodes that are formed on the upper and lower surfacesof the nanocomposite material, and can be also used as and image formingmeans of a flat panel display with a backlight unit. Further, thenanocomposite material of the present invention can be applied as asensing device that detects a toxic gas, for example, by takingadvantage of the property that a range of wavelengths of projected lightis changed based on the ambient environment.

Meanwhile, the matrix material employed in the nanocomposite materialaccording to the present invention can include a metal oxide other thansilica (SiO₂), such as titanium oxide (TiO₂). Precursor materials fortitanium oxide are known; and a process of manufacturing the titaniumoxide is substantially identical to that disclosed above.

In the present invention, in order to prevent cracks due to weakening ofthe binding force between a host matrix and nanoparticles incorporatedtherein in a dispersed state during the drying procedure formanufacturing the nanocomposite material, a surface modifying process inwhich the surface of the nanoparticles is coated with a componentmaterial of the matrix is incorporated prior to the drying procedure.Thus, by preventing macro- or micro-scale cracks of the nanocompositematerial, the resulting stable light-emitting properties can maintainand enhance light intensity. In particular, the nanocomposite materialaccording to the present invention can structurally prevent lightintensity from decreasing due to crystallization by applying a metaloxide matrix.

Although the present invention has been described herein with referenceto the foregoing exemplary embodiments, these exemplary embodiments donot serve to limit the scope of the present invention. Accordingly, itwill be understood by those of ordinary skill in the art that variouschanges in form and details may be made therein without departing fromthe spirit and scope of the present invention as defined by thefollowing claims.

1. A nanocomposite material, comprising: a plurality of quantum dotscoated with a metal oxide; and a matrix of the metal oxide immobilizingthe quantum dots that are dispersed therein.
 2. The nanocompositematerial of claim 1, wherein the matrix is formed from a networkstructure of the metal oxide.
 3. The nanocomposite material of claim 1,wherein the metal oxide is SiO2 or TiO2.
 4. The nanocomposite materialof claim 1, wherein the plurality of quantum dots comprise a Group II-VIsemiconductor compound selected from the group consisting of CdSe, CdTe,CdS, ZnSe, ZnTe, ZnS, InP, GaP, and GaInP₂.
 5. The nanocompositematerial of claim 1, wherein the plurality of quantum dots comprisemetal particles selected from the group consisting of Au, Ag, Fe, andCo, or an oxide particle of the selected metal.
 6. The nanocompositematerial of claim 1, wherein the metal oxide used to coat the quantumdots is substantially the same as the metal oxide used in the matrix.